[SecretManagement] Have a SecretManagement extension `dotnet new` template

[TylerLeonhardt]

Summary of the new feature/enhancement

This is basically the C# version of #84

We have a dotnet new psmodule in the PowerShellStandard repo

We should have a template for Secret Management extensions. This will help a ton with adoption because we can bootstrap the user.

Proposed technical implementation details (optional)

• A dotnet new template with the files listed in the C# vault extension module section of @PaulHigin's blog post:
  https://devblogs.microsoft.com/powershell/secrets-management-module-vault-extensions/

• Maybe a CodeTour if we're feeling really flashy?